I would hope it's extremely rare. Poor engineering and/or use of sub-standard materials is the real culprit. Our underbelly is exposed and was easy to examine tank integrity, unless something catastrophic were to occur I could see no reason for tanks to separate.
I can understand wanting to know if this is indeed a common issue, having this occur during transit would be a nightmare. Take a look at your rig's set-up, it would be worth the peace of mind even if you had to remove some of the underbelly's cover to do so.
I, for one use my freshwater tank as intended, serving it's purpose of transporting water wherever our RV travels take us. If I had any reservations about my rig's ability to do so, I would certainly get rid of it in return for one that could perform such a routine task.